1. Field of the Invention
The present invention relates to hydraulically driven tools or instruments, such as for example chain saws, and in particular to a drive unit for such tools or instruments.
2. Description of the Prior Art
Hydraulically driven chain saws have been used for some years. Connections are provided on the chain saw for attachment to the feed and return hoses or pipes of a hydraulic power source.
The hydraulic power source which is most often used is a so-called hydraulic power-pack, that is a free-standing hydraulic power system which may be transported on a vehicle to the desired location of use, and then used to drive a variety of different tools or instruments.
Another hydraulic power source which has been used is the hydraulic system of a prime mover, such as for example a conventional agricultural tractor.
The use of the hydraulic system of a conventional agricultural tractor as a source of hydraulic pressure presents certain difficulties when operating particular types pf tools such as chain saws. The principal difficulty relates to the fact that the two hydraulic connections from the agricultural tractor for feed and return of hydraulic fluid, unlike those of the hydraulic power-pack, are identical, and that the tractor hydraulic system may be driven in either direction. Hydraulic chain saws and other such hydraulic devices are usually designed to have hydraulic fluid flowing in one direction only. As a result, from time to time the hydraulic chain saw may be connected the wrong way around to the hydraulic system of the agricultural tractor and may be damaged.
The circumstances in which such damage most often arises will now be described more particularly. The chain saw comprises a hydraulic motor driven by hydraulic fluid pressure. The drive from the chain saw motor, which is contained in a sealed housing, is transmitted by means of a drive shaft to the chain saw sprocket which is located outside the housing. The drive shaft passes through a seal on the wall of the housing. The hydraulic fluid, as well as serving its principal purpose of driving the chain saw motor, also lubricates the chain saw motor and the drive shaft and lubricates the region of the seal. A bleed pipe leads from the region of the seal to the return side of the motor so that lubricating hydraulic fluid may be re-circulated. The seal is subject to the lower hydraulic pressure which prevails at the fluid return side of the motor and is specified to withstand that lower pressure. If the chain saw is inadvertently connected the wrong way around to the hydraulic system of the tractor the seal is directly subjected to the higher feed pressure and the seal or the housing itself may become damaged. Alternatively, if the chain saw feed pipe is connected correctly but the chain saw return pipe is inadvertently not connected, or alternatively becomes blocked, then a back pressure may build up at the return side of the bleed pipe and the feed pressure is applied indirectly to the seal.
An object of the present invention is to provide a hydraulic drive unit which may be attached to the hydraulic system of a conventional agricultural tractor or other prime mover without the risk of damage. such a drive unit could be used to drive a variety of different tools, a chain saw being just one example. Such a drive unit would allow hydraulic tools to be used much more widely and to be generally more versatile, as agricultural tractors are more numerous than hydraulic power-packs and are more mobile.